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GSSP for the Sheinwoodian Stage (base of the Wenlock Series) Hughley Brook, Shropshire

Locality:Section in banks of and adjacent to Hughley Brook, 200 m SE of Leasows Farm and 500 m NE of Hughley Church (grid reference SO 5688 9839).

Lithology and fossil constituents

Bassett et al. (1975) defined nine lithological units at the GSSP, labelled A to I. Bassett et al. (1975) listed a diverse shelly macrofauna from the top 10 m of the Purple Shales, but did not indicate which species occur in the units A to F, only that brachiopods are ‘sporadic’ in Unit A. The earliest Sheinwoodian at Hughley Brook contains the brachiopods Dicoelosia biloba, Leangella segmentum, Atrypa reticularis, Eoplectodonta duvalii, Cyrtia exporrecta, Atrypina barrandii, Eospirifer radiatus, Streptis grayii, Isorthis elegantulina, Resserella sabrinae, Glassia obovata, Dictyonella capewellii, Dalejina sp. and indeterminate rhynchonellids. Also recorded were uncommon trilobite fragments, common bryozoan debris, small solitary corals and rare bivalves and gastropods.

Mabillard & Aldridge (1985) discussed the distribution of microfossils at the GSSP, describing the assemblages as ‘rich’. Range charts were provided for acritarchs (and a spore), chitinozoans, conodonts and ostracodes. Mullins & Aldridge (2004) subsequently revised the chitinozoans from the section. A revision of the conodonts is ongoing (Peep Männik, pers. comm. to David Loydell).

Conodont Biostratigraphy

The GSSP level occurs within the Pterospathodus amorphognathoides amorphognathoides conodont zonal group, between acritach zone 5 and the last occurrence of P. am. amorphognathoides (Melchin et al ., 2012). The GSSP level is conveniently placed at the base of the Buildwas Formation. At the Hughley Brook GSSP this level was thought to correlate “with the base of the Cyrtograptus centrifugus graptolite Biozone” (Martinsson et al. 1981). This was based on the occurrence of Monoclimacis aff. vomerina and Pristiograptus watneyae higher in the Buildwas Fm and species indicative of the M. greistonesises and M. crenulata zones lower in the Purple Shales Fm (Melchin et al ., 2012). However, the GSSP section lacks graptolites in both the Purple Shales Formation and overlying Buildwas Formation (other than a few indeterminate fragments in palynological residues; Mullins & Aldridge 2004, p. 745) and thus direct section-based evidence of this conodont to graptolite relationship is lacking.

Work on revising the Hughley Brook conodonts is ongoing. Loydell (2008) said “In the conodont biozonation, the base of the Sheinwoodian lies very close to the base of the Upper Pseudooneotodus bicornis Biozone, equivalent to Ireviken Event Datum 2”, but noted that this was not immediately apparent from examination of Mabillard and Aldridge’s (1985) text-figure 6. Jeppsson’s (1997, p. 95) comment, upon which Loydell’s (2008) statement was based, resulted from his examination of the Mabillard and Aldridge collections held at the University of Leicester. Peep Männik has also examined the Mabillard and Aldridge collections (and is engaged in further study of them). His view (pers. comm., 2 September 2005 to Loydell) was that Datum 2 occurs somewhere between Mabillard and Aldridge’s samples 36 and 39, i.e. between 12.5 cm below and 16.1 cm above the base of the GSSP level at Hughley Brook . It seems likely that the Ireviken Datum 2 and the base of the C. murchisoni Biozone occur within 200 kyr of the GSSP level at Hughley Brook (Melchin et al. 2012), either of which could provide a suitable correlation proxy for the base of the Sheinwoodian.

Chitinozoan Biostratigraphy

Work on chitinozoans and conodonts (summarized in Loydell 2008) has indicated that the GSSP level correlates with a higher stratigraphical level in the graptolite biozonation than originally inferred- near the base or lying within the Cyrtograptus murchisoni Biozone. Mullins & Aldridge (2004) chitinozoan zonation from the section placed the inferred Sheinwoodian up to 1.211 m above the base of Bed G in the Buildwas Formation. The underlying Purple Shales Formation yields low diversity assemblages and only two species and three genera were confidently identified, and these were described as “not diagnostic of any chitinozoan biozone”. In Unit F (representing the top 15 cm of the Purple Shales Fm) are there diverse, biostratigraphically useful chitinozoan assemblages. These indicate the upper part of the Margachitina margaritana Biozone. The lowermost part of the Buildwas Formation was also assigned by Mullins & Aldridge (2004) to the upper Margachitina margaritana Biozone . The chitinozoan Cingulochitina bouniensis has its first occurrence 25.6 cm above the base of the Buildwas Fm marking the base of the bouniensis Biozone. The uppermost three samples of Mullins & Aldridge(2004) were assigned to the Salopochitina bella Biozone, the base of which is 82.5 cm above the base of the Buildwas Fm, implying the section is condensed.

Integrating the results from the Hughley Brook chitinozoan work, with studies on the chitinozoans from graptolitic sections on the Welsh Basin margins (Verniers, 1999; Mullins & Loydell, 2001, 2002) enabled Mullins & Aldridge (2004) to suggest a limited stratigraphical range for the GSSP level at Hugh Brook- within the upperpart of the Cyrtograptus centrifugus to lower part of the Cyrtograptus murchisoni graptolite biozonal interval. This is approximately one graptolite biozone younger than inferred in the original GSSP description.

Sequence stratigraphy

Loydell’s (1998) eustatic sea-level curve for the early Silurian showed a major fall in sea-level in the late Telychian (commencing mid Cyrtograptus lapworthi Zone). In many places (e.g. the East Baltic, Spain) there is an unconformity at this level, with C. murchisoni Biozone strata overlying those assigned to the C. lapworthi Biozone.

The latest Telychian and earliest Sheinwoodian witnessed a eustatic sea-level rise and at many localities world-wide transgressive strata yield C. murchisoni Biozone graptolites. In parts of Wales (e.g. the Builth Wells area; Zalasiewicz & Williams, 1999), the C. murchisoni Biozone is condensed as a result the high sea-levels starving sediment supply. This is in accord with the condensed chitinozoan biozones and Pterospathodus amorphognathoides conodont biozones at the Hughley Brook GSSP (Mullins & Aldridge, 2004; Mabillard & Aldridge, 1985).

Isotope stratigraphy

A descriptive account of δ13Ccarb values from the Hughley Brook section was presented by Loydell (2011), but without details. The much longer record through correlative strata is that of Hughes & Ray (2016) in which they show the Purple Shales- Buildwas Fm boundary occurs within a plateau in δ13Ccarb values immediately prior to the start of the early Sheinwoodian isotope excursion and the S. bella chitinozoan Biozone.

In the Banwy River section (Loydell & Cave 1996; Mullins & Loydell 2001), the onset of the early Sheinwoodian excursion is within the uppermost part of both the C. murchisoni and M. margaritana biozones (Cramer et al. 2010), which suggest like other data the GSSP level at Hugh Brook is probably within the lower part of the C. murchisoni graptolite Biozone. A single bentonite, 10 mm thick, has been recorded from 2.84 m above the base of the Buildwas Fm (Bassett et al.1975), but has so far not been radiometrically dated.

Site Visits: Strata at the GSSP are generally poorly exposed and thus the section can only be studied after excavation, which must be conducted after consultation with Natural England who are responsible for the locality. From a site conservation and management perspective, this lack of exposure may be a good thing since it enables Natural England to monitor new work being undertaken on the section and prevents casual visits and potential damage to the limited exposure present. Hence, any visit to the GSSP needs to be planned many months in advance.